The present invention relates to damping mechanisms and vibration isolation mechanisms. More particularly, the present invention pertains to a high-strength, compact, magnetorheological-fluid-modulation-damped vibration isolator.
The use of magnetorheological (MR) fluid in a damping device allows for the controlled variance of device damping as a function of the strength of a magnetic field induced into a controlled or valved region of the MR fluid. Coil electromagnets, permanent magnets, or a combination of magnet types are used as the means for magnetic field creation. The use of coil electromagnets allow for the variance of the magnetic field with the variance of the electrical signal amplitude applied to the coil. Many devices exist within the prior art that take advantage of this smart material capability of MR fluids.
Problems present in MR fluid damping devices of the prior art include fluid leakage and rapid seal wear in devices incorporating dynamic type sealing, i.e. where surfaces slide over one another such as a piston rod sliding through a concentric lip seal. The maintenance of good lateral alignment of the moving components of the damper relative to the fixed components and the support of off-axis moment loading is also problematic within devices of the prior art. Tighter seals and bushings are often used for improved alignment and moment support but cause greater friction loads and stiction effects between the moving components. Devices of the prior art have thereby been relatively intolerant to off-axis moment loading.
An example of the prior art usage of magnetorheological fluid in a damping device where dynamic seals are relied upon is seen in U.S. Pat. No. 5,277,281. Therein a damper assembly is filled with MR fluid and an electromagnetic coil is contained within the damper piston. The viscosity of the MR fluid flowing past the piston is varied by varying the magnetic flux around the piston by means of an electromagnetic coil mounted within the piston. In an alternative embodiment of that patent, two tubes are utilized, one concentric to the other, wherein a piston forces fluid out of the inner tube and into the outer tube across a valved area controlled by a stationary coil at the end of the tubes. In both these embodiments, dynamic sealing is relied upon around the piston shaft.
Bellows type sealing and relative motion provision within a fluid damper have been described in U.S. Pat. No. 4,815,574. Therein a bellows surrounds a piston shaft and thereby prevents damping fluid from contacting the piston shaft at its protrusion from the surrounding damper cylinder. Lateral alignment of the piston shaft and guidance within the cylinder are still, nevertheless, accomplished with a bushing at the end plate through which the piston shaft passes. Friction forces and stiction develop at this bushing, and lateral alignment of the piston within the cylinder is controlled largely by the lateral forces developed on the piston by the cylinder wall, further adding to friction and stiction effects. Additionally, this device does not provide for static load carrying except at the end of travel points of the piston.
Often damping mechanisms of the prior art offer damping capability only and do not provide static load carrying capability. This is the case with the patents described above which require the dampers to be placed in parallel with static load carrying, vibration motion isolating members, such as coil or leaf springs or elastomeric mounts. The support structure for a payload thereby requires significantly greater space and attachment hardware than that afforded by a single device offering both damping and load carrying integrally.
Dampers which do provide integral static load carrying capability commonly use elastomeric elements in the primary load path of the device. U.S. Pat. No. 5,398,917 shows an example of a MR fluid damper incorporating an elastomeric element to serve as a spring for vibratory motion isolation. U.S. Pat. No. 5,284,330 describes an MR fluid damper wherein elastomeric elements are used to allow the relative motion between a piston and its surrounding cylindrical fluid chamber. Similarly, U.S. Pat. No. 5,492,312 uses elastomerics to allow relative motion of a central shaft and piston relative to a surrounding fluid confining cylinder. In these devices the elastomeric elements do provide a static load path within the device. The drawbacks with the use of elastomeric elements, however, are the non-linear load/deflection characteristics imparted to the device and the relatively low strength capabilities of the elastomeric elements which limit the static load carrying capability of the device.
Applications in aerospace payload support commonly require damper and vibration isolator mechanisms to have as low a profile as possible so to minimize the lengthening of the overall spacecraft structure. It is often desirable to insert a damping and vibration isolation support mechanism within the existing interface of a payload and its support structure. The desire for low profile, compact structures adds value to devices which maximize the damping force effected for a given length of damper. The elimination of stiction in device performance also becomes of premium value where precise motion control and positioning of a payload is desired. Mechanical robustness, reliability, and predictability of performance are additional qualities required of airborne devices.
Notwithstanding the many devices of the prior art utilizing magnetorheological fluid for damping, there remains a need for a device that combines within a single, low profile, compact package, the wide range of damping controllability of a magnetorheological fluid damper along with high strength and optionally linear-elastic load carrying capability accompanied with substantial vibration and shock load isolation. The device should also avoid the stiction and high wear sealing problems associated with dynamic seals prevalent in MR fluid devices of the prior art. The invention described herein provides for such a device.